Exhaust systems of modern internal combustion engines, particularly in the case of motor vehicles, are regularly provided today with catalytic converters for the catalytic decomposition of harmful exhaust gases. In order for the catalytic converters to function well, it is necessary to feed air and fuel to the engine in a predefined proportion. The engine controls provided for this purpose are connected on their input side to a so-called lambda probe whose signals represent the composition of the exhaust gas and thus enable the engine control to adjust the ratio of fuel and combustion air in a manner optimal for the catalytic converter.
In the case of the measuring sensors indicated at the outset, the ceramic sensor member is constructed at least region-wise as a solid electrolyte member which is conductive for oxygen ions. This conductivity is utilized to generate an electrical signal correlated to the oxygen concentration of the gas to be analyzed.
Since the effect of the conductivity for oxygen ions is strongly temperature-dependent, the sensor member must be heated during measuring operation in order to be able to generate evaluable signals. Typical operating temperatures of the sensor member lie between 300° C. and 450° C.
German Patent No. 30 35 608 describes a measuring sensor of the type indicated at the outset. In that case, the sensor member is constructed as a ceramic solid electrolyte tube which has heating arranged on the outside wall and is preferably accommodated in a protective sleeve having openings for the entrance of the gas to be measured.
German Patent No. 29 42 494 likewise describes a measuring sensor whose sensor member is constructed as a ceramic solid electrolyte tube. The heating is arranged on the inner side of a protective housing which accommodates the sensor member and is provided with an opening for the gases to be analyzed. Thus, the housing forms a heating and protective sleeve for the sensor member which is heated indirectly in this configuration.
German Patent No. 36 28 572 describes a tubular ceramic sensor member which is made of electrically conductive ceramic material on its measuring-gas-side end, and the remainder of which is made of electrically insulating ceramic material, the region of electrically insulating ceramic material bearing electrical heating conductors on the exterior.